The present invention relates to a radio telephone with a channel switch for tuning the radio telephone to one of a certain number of particular radio channels selected from a larger number of available radio channels.
In such radio telephones, which are exemplified by citizens band radios, each channel is associated with its own discrete oscillator frequency which is furnished by a phase locked loop (PLL) oscillator circuit containing a frequency divider having a dividing ratio which can be digitally switched by means of the channel switch.
Multichannel radio telephones must be able to be switched to any one of a number of different radio channels which are arranged according to a frequency pattern. Since only very narrow frequency tolerances are permissible for the individual channel frequencies, the frequency determining elements of the oscillator circuits can only be quartz crystals.
For example, there are units in which one quartz crystal is provided for each radio channel and in which the quartz crystal associated with the desired radio channel can then be switched on by means of a single pole channel switch. It might be noted, in this connection, that there exist radio telephone networks employing, for example, radio telephone units having a ten to twenty channel capacity in which a large region is divided into spatially limited areas and each area has assigned to it a different series of radio channels. These radio telephone units must thus be equipped with different sets of quartz crystals, the set selected depending on the area in which they are to be employed.
In order to avoid the necessity for having available a quartz crystal for every one of the possible frequencies, and instead to be able to use one quartz crystal as a reference standard, oscillations at the frequencies associated with a plurality of channels in a multichannel radio telephone can be generated by means of a multichannel oscillator employing digital frequency division. Such an oscillator in part of a PLL oscillator circuit and includes an oscillator which is controllable by means of an electrical voltage, a frequency divider producing a dividing ratio which can be digitally set by means of the channel switch of the radio telephone and a phase comparison circuit. While a first input of the phase comparison circuit receives a reference frequency signal which is derived from the quartz crystal, the second input receives a signal represented by the oscillator output reduced in frequency by means of the frequency divider. At one output, the phase comparison circuit emits a voltage whose value depends on the phase difference between the two frequencies and which serves to regulate the oscillator.
Since the frequency divider must be digitally controllable, the channel switch of the radio telephone may be designed, for example, as a coding switch, as disclosed in German Offenlegungsschrift [Laid-Open Application] No. 23 04 772. Before the radio telephone is used for the first time, such a coding switch must then be coded for the intended series of radio channels, i.e., it has to be manually set to a certain switching combination for each channel. Such a channel switch has a relatively complicated structure and programming of the switch is a time consuming operation.
For multichannel radio telephones with, for example, 120 different radio channels, it has been proposed to employ two BCD coding switches in conjunction with a recoder.